Granular nitrocellulose manufacture

ABSTRACT

IN THE PROCESS FOR THE PRODUCTION OF GRANULAR NITROCELLULOSE BY TREATING FIBROUS NITROCELLULOSE IN A BATH OF WATER AND AN ACTIVE SOLVENT, THE IMPROVEMENT COMPRISING ADDING DISPERSANT TO THE BATH.

United States Patent O U.S. Cl. 260-223 14 Claims ABSTRACT OF THE DISCLOSURE In the process for the production of granular nitrocellulose by treating fibrous nitrocellulose in a bath of water and an active solvent, the improvement comprising adding dispersant to the bath.

BACKGROUND OF THE INVENTION Nitrocellulose or cellulose nitrate is used extensively in lacquers, plastics, and propellant compositions. Generally, nitrocellulose is supplied to the user in the form of mechanically compacted masses of fibers that are similar in physical form to the cellulose fibers from which the nitrocellulose was made. Because dry fibrous nitrocellulose is exceedingly flammable, commercial nitrocellulose is supplied to the market wet with a liquid that is a nonsolvent for nitrocellulose in order to minimize fire hazards during handling, shipping, storage, and further processing. The most commonly used wetting liquids are water, the lower molecular Weight alcohols such as ethanol, isopropanol, and butanol and hydrocarbons such as benzene and toluene. The solvent-wet fibrous nitrocellulose, however, is somewhat difiicult to handle, since it tends to agglomerate and form slow-dissolving masses when added to an active solvent, as in the preparation of lacquers. Further, the density of the fibrous nitrocellulose, even when heavily compressed during packing, is relatively low, thereby requiring a large number of shipping containers.

Various means have been employed to overcome the handling problems associated with use of commercial liquid-wet fibrous nitrocellulose most of which involve some type of mechanical compaction of the fibrous nitrocellulose which subsequently is broken into smaller particles.

Another means of compacting fibrous nitrocellulose, disclosed in U.S. Pat. 670,346, involved changing it to a structureless coherent material by subjecting the nitrocellulose to the action of solvents diluted with a portion of non-solvent liquid. For example acetone-with-water and nitrobenzene-with-alcohol were disclosed as suitable pairs of liquids for use in the process. It is known that below the compatibility point e.g., the concentration at which nitrocellulose just becomes soluble) nitrocellulose swells, and the ratio of acetone to water imbibed by the nitrocellulose is higher than that existing in the bulk liquid, in other words, the nitrocellulose selectively absorbs the active solvent. Gelatinization, but not dispersion, of fibrous nitrocellulose occurs if there is more than about 12% of water in acetone.

The phenomenon of preferential absorption by nitro- 3,563,977 Patented Feb. 16, 1971 cellulose of an active solvent from non-solvent liquid/ solvent solution, with gelatinization of the nitrocellulose, has been used to make a free-flowing, fast-dissolving, high bulk density nitrocellulose product in a water-based system, by use of an active solvent for nitrocellulose, which solvent formed a minimum boiling azeotropic mixture with water. One such procedure for making dense particulate nitrocellulose, but without the limitation of the minimum boiling azeotropic mixture, was incorporated into a process for casting double base propellant grains described in U.S. Pat. No. 2,946,673. A similar process for making hydrocarbon-wet granules of nitrocellulose is shown in U.S. Pat. No. 3,284,253. These disclosures point ed out that from an economic viewpoint in making granules from a nitrocellulose-solvent-water slurry, it is desirable to employ slurries containing as much fibrous nitrocellulose as practicable, and the practical upper concen tration limit of nitrocellulose in the slurry is governed by the ability to agitate the slurry effectively. Generally nitrocellulose-liquid slurries containing about from 5 to 20% by weight of fibrous nitrocellulose were used in these processes. A higher nitrocellulose content in the slurry was possible only by comminuting the fibrous nitrocellulose for example, by jordaning or grinding in a ball mill before subjecting the fibrous nitrocellulose to the solvent-densification process. When such comminuted fibrous nitrocellulose is employed to make smooth, hardened, densified granules of nitrocellulose, the process is more costly to operate and is less capable of producing the larger size particles of densified nitrocellulose which often are preferred.

A further requirement for success of these processes was to maintain vigorous agitation in the slurry from the moment that the nitrocellulose was contacted by the liquid until the softened nitrocellulose particles were hardened by removal of substantially all of the active solvent. Agitation prevented substantial agglomeration of the softened nitrocellulose particles into lumps, and had to be sufiiciently vigorous at all times to keep the nitrocellulose uniformily distributed throughout the slurry.

The processes previously used frequently had to employ solvents other than acetone to obtain satisfactory results. In some cases, such as those processes requiring a minimum-boiling azeotrope, acetone could not be used at all, and a higher-cost solvent such as methyl ethyl ketone was necessary.

Therefore, although the granular products produced by these processes were often satisfactory, the cost of production was increased by the higher power requirements of vigorous stirring, lower nitrocellulose/liquid ratio, and the fact that maximum efiiciency could not be obtained by the use of low-cost active solvents.

SUMMARY OF THE INVENTION The instant invention provides an improved process for the production of granular nitrocellulose characterized by greater efliciency and economy than have heretofore been obtainable. This process permits exceptionally high concentrations of fibrous nitrocellulose in the nitrocellulose/water/solvent slurry, requires substantially less vigorous stirring to satisfactorily disperse the nitrocellulose in the aqueous medium, and operates very elfectively with extremely low-cost active solvents.

Specifically, this invention provides an improvement in the process for preparing granular nitrocellulose which comprises dispersing fibrous nitrocellulose in an aqueous medium containing a volatile, water-miscible solvent for said nitrocellulose in an amount sufiicient to soften but not dissolve said nitrocellulose, and distilling off said solvent, the improvement which comprises incorporating in said medium dispersant selected from the group consisting of at least one of (a) ammonium and alkali metal hydroxides and (b) water-soluble salts of polybasic acids having a first dissociation constant of less than 2X10- and more than one equivalent per mole of said acid, of said hydroxides.

DESCRIPTION OF PREFERRED EMBODIMENTS All ingredient ratios, concentrations, percentages, and the like expressed herein are parts by Weight unless otherwise specified.

The process of the instant invention is applicable to fibrous nitrocellulose which has been subjected to the usual stabilization and viscosity-control treatments. The nitrogen content of the nitrocellulose can be about from 10.5% to 13.7% but usually ranges about from 10.7 to 12.7%, and the viscosity level, which varies according to the molecular weight, ranges from about 1.5 seconds for a solution in a /25/20 mixture (by weight) of toluene/2B specially denatured alcohol/ 85% ethyl acetate to over 2000 seconds for a 12.2% solution in the same solvent mixture (ASTM Standard Test No. D301- 33). The stabilized fibrous nitrocellulose usually is water wet and preferably contains not more than 35% of its weight of water. While this limit on water content of the nitrocellulose is not critical to the process of the instant invention, such a limit permits easy adjustment of the solvent/nitrocellulose ratio for operation of the process and, additionally, permits reuse of solvent that is recovered at about -65% strength by steam-stripping the solvent from the process slurry Without special provision for fractional distillation during stripping of the solvent, in the manner described in the examples which follow.

The weight of water in the process slurry, including water which is introduced with the fibrous nitrocellulose, comprises about from 100 to 900 parts and preferably about from 100 to- 200 parts per 100 parts of fibrous nitrocellulose.

Active solvents which can be used in the process of the instant invention include those organic solvents having boiling points below 100 C. or which are steamdistillable below 100 C., for example esters such as methyl acetate, ethyl acetate, propyl acetate, butyl acetate, and isopropyl isobutyrate; and ketones such as acetone, methyl ethyl ketone, and methyl isobutyl ketone. Ethyl acetate, acetone, and methyl ethyl ketone are preferred active solvents, and acetone is especially preferred because of its particularly low cost and ready availability. The weight of solvent generally comprises about from 60 to 300 parts per 100 parts of fibrous nitrocellulose. The upper limit for the amount of active solvent is determined by the consideration that in no case should there be sufilcient solvent to dissolve the nitrocellulose in the water-solvent mixture. The ratio by weight of active solvent/nitrocellulose preferably is maintained as low as possible to reduce the amount required to be distilled olf. Operation at ratios below 1.0 is entirely practicable, ratios of about 0.6-0.8 being especially preferred. Processes previously used employed solvent/nitrocellulose ratios of about from 1.02.0 in order to form granules of nitrocellulose.

The optimum weights of solvent and of water used to obtain a high quality of granular nitrocellulose in the process of this invention vary according to the type of nitrocellulose and the particular active solvent. The proc- 4 ess of this invention gives satisfactory results with slurries which initially contain about 30-35% of fibrous nitrocellulose, far greater percentages than have heretofore been possible. For example, a very satisfactory ratio of ingredients using a high nitrogen medium viscosity nitrocellulose is about 70 parts of acetone and 130 parts of water per 100 parts of nitrocellulose. A satisfactory ratio of nitrocellulose/water/ active solvent is considered to be one that gives a dense, granular nitrocellulose product that is substantially free of fibrous nitrocellulose and has a particle size largely in the range of 3-80 mesh (US. Standard Sieve series). The particle size also is affected by the grade of fibrous nitrocellulose which is being granulated, the effectiveness of the agitation, and the tem- V v the use of a dispersant permits processing a slurry of fibrous nitrocellulose containing a higher nitrocellulose/ liquid ratio, but which requires less power for agitation of the slurry during swelling and coalescence of the nitrocellulose and during the subsequent solvent-stripping period to obtain granulated nitrocellulose of the preferred range of particle sizes. Additionally, the presence of a dispersing agent in the liquid medium permits use of acetone as the active solvent, and in fact helps to make acetone the preferred active solvent for the process of the invention.

Dispersing agents which can be used in the instant invention include ammonium hydroxide and alkali metal hydroxides such as sodium hydroxide, potassium hydroxide, and lithium hydroxide. Other dispersants include water-soluble salts formed by reaction of (a) polybasic acids that have a first dissociation constant of less than 2 l0 for example, azeleic, boric, carbonic, citric, maleic, oxalic, ortho phosphoric, orthophthalic, pyro phosphoric, tartaric, and silicic acids; with (b) more than one equivalent, per mole of said acid, of a base selected from ammonium and alkali metal hydroxides. Representative salts of acid organic phosphates which can be used as dispersants include (1) water-soluble ammonium and alkali metal salts of acid polyoxyethylene nonylphenol phosphates, such as Gafac LO-529, PE-510, RE6l0, RS-6l0 and RS-7l0; (2) water-soluble ammonium and alkali metal salts of acid alkyl phosphates, for example, Zelec NE; and (3) water-soluble ammonium and alkali metal salts of acid fiuoroalkyl phosphates such as Zonyl S-l3. The acid organic phosphates are all derivatives of phosphoric acid in which the phosphoric acid is partially esterified by an organic OH-bearing moiety. Of the above acid phosphates, Gafac LO-529 is supplied as a sodium salt, while the remainder are supplied as acid esters, the remaining acidic hydrogens of which are then neutralized to a pH of about 7 by ammonium or alkali metal hydroxides to form the dispersing agents indicated in (1), (2) and (3) above. Of all dispersing agents indicated above, trisodium phosphate has been found particularly effective.

The amount of dispersant used generally varies about from 0.02 to 5.0 part by weight per 100 parts of nitrocellulose, and it has been found that 0.03 to 0.5 part of dispersant per 100 parts of nitrocellulose are especially effective. Too little dispersant results in inadequate softening and coalescence of the fibrous nitrocellulose or higher power requirements for agitation, whereas use of excessive amounts of dispersant results in a densified product which contains a substantial proportion of very fine particles which pass an -mesh sieve. It has also been found that somewhat less dispersant is required if higher viscosity fibrous nitrocellulose is used.

Temperature is not critical to the process of the instant invention, and can vary from room temperature to the atmospheric boiling point of the slurry mixture. If the process is carried out at or near the boiling point of the slurry, a closed vessel with reflux condenser is preferably employed to contain the active solvent, and a somewhat lower solvent/nitrocellulose ratio can be used effectively. On the other hand, if the process is carried out at room temperature or somewhat above, a higher solvent/ nitrocellulose ratio is required. A temperature of about 50 C. during the swelling and coalescence of the fibrous nitrocellulose is preferred, and is especially preferred if acetone is the active solvent that is used in the slurry mixture. The temperature of the slurry is increased to about 100 C. at atmospheric pressure during stripping and recovery of the solvent, while the nitrocellulose particles are being hardened. In the interest of economy of operation, the heating is continued until the slurry is substantially solvent free, after which the granular nitrocellulose is separated from the aqueous slurry.

The nitrocellulose slurry should be agitated in the process of the instant invention, but less power is required than in nitrocellulose-densification processes heretofore known which did not employ a dispersant. Agitation is maintained in the aqueous slurry from the time the fibrous nitrocellulose contacts the solvent until the particles of softened nitrocellulose have been hardened by removing the active solvent from the slurry, e.g., by distillation. Agitation is sufficiently vigorous to keep the nitrocellulose dispersed throughout the liquid at all times.

The specific details of the operation of the process are not critical to the instant invention, and can vary widely. depending on the type and grade of starting materials used and the size and quality of nitrocellulose granules desired. In general, however, it is preferred that the fibrous nitrocellulose be added to a vigorously stirred mixture of Water, active solvent, and dispersant at a temperature of about 50 C. The softening and aggregation of the fibrous nitrocellulose begins almost at once and is complete within a few minutes after all the fibrous nitrocellulose has been added to the slurry. Stirring is continued While the temperature of the slurry is raised by heating the mixture, or by sparging with steam, to drive off the active solvent which is condensed and recovered, after which agitation can be stopped. The granular nitrocellulose is separated from the water slurry by any suitable means such as by allowing the granular nitrocellulose to drain freely, or by filtration, centrifugation, or the like. The water-wet granular material is washed with fresh water at normal temperatures and then, if desired, can be wet-screened or otherwise classified to separate any unusually large or small particles which are not desired in the product. The water-wet product easily can be packed for shipment at net weights per container considerably above those achieved by compressing the usual fibrous cellulose.

Possible alternate procedures include, for example, first agitating fibrous nitrocellulose in water containing the dispersant, after which the active solvent is added to the slurry, and the process then carried on as before.

In another alternative, the vigorously agitated slurry of softened nitrocellulose in the mixture of water, acetone, and dispersant is diluted by gradual addition of a substantial quantity of water, whereupon the aggregates of nitrocellulose are hardened into smooth, dense particles which can be separated from the slurry, washed, and processed further, as indicated above. This procedure is generally less desirable because the active solvent is lost if the liquid is discarded after separating the granular nitrocellulose, or a much larger volume of liquid must be distilled to recover the active solvent for reuse than is the case for the preferred practice of the process of the invention.

The nitrocellulose product obtained by the process of this invention is in the form of free-flowing, dense particles, generally smoothly rounded in form, though not necessarily spherical in shape and free of fibrous nitrocellulose. The particle size and density of the granular nitrocellulose will vary with the types and proportions of the solvents and nitrocellulose used in the process of the instant invention as well as the other process variables indicated above. However, in general, the nitrocellulose product will have a dry bulk density of about from 33 to 51 pounds/cubic foot (0.53-0.82 g./ml.), and a particle size such that not more than 20% of the particles fall outside the size range represented by through 3- and on -mesh US. Standard sieves.

The following specific examples further illustrate the process of the instant invention.

EXAMPLE 1 A /2 second lacquer grade nitrocellulose containing 1l.8-l2.2% nitrogen and having a viscosity of about 14 seconds (aluminum ball) at 20% concentration in the standard test solution (grade HB-14) is used in this example. The Water-wet, stabilized, fibrous nitrocellulose g. dry weight) is added with agitation to a mixture containing 70 g. of acetone, 0.08 g. of Na PO and additional Water, the total water content of the combined mixture being g. The temperature of the mixture is 50 C. The slurry is agitated with a twisted blade-type agitation driven by a Gast Air Motor. Uniform dispersion of the nitrocellulose is obtained at a motor speed of 850 r.p.m., using an air pressure of 20 p.s.i.g. This slurry contains 33.3% of nitrocellulose. Swelling and coalescence of the fibrous nitrocellulose begins at once. Agitation is continued while the mixture is heated by injection of live steam until the acetone is stripped from the agitated slurry. During this period of solvent stripping, the densi-fication of the nitrocellulose is completed. The granular nitrocellulose is separated from the residual liquid by centrifugation (total volatiles content about 30%), then is washed twice by slurrying in isopropanol (2/1 isopropanol/nitrocellulose) to obtain isopropanol-wet granular nitrocellulose containing about 5 parts of water per 100 parts of dry nitrocellulose. After drying, the granular nitrocellulose all passes through a screen having /8 openings, is very free flowing, is free of fibrous nitrocellulose, and has a bulk density of about 45 pounds/cubic foot.

EXAMPLE 2 The procedure of Example 1 is. repeated, except that the 0.08 gram of Na PO is omitted. The motor speed required to obtain a comparable degree of dispersion is 1650 r.p.m., using an air pressure of 65 p.s.i.g.

EXAMPLE 3 The procedure of Example 1 is repeated except that no dispersant is added to the water-acetone solution before adding the nitrocellulose. The conversion of fibrous nitrocellulose to granular nitrocellulose is substantially decreased, the product consisting of 56% of granular nitrocellulose, through 3- and on IO-mesh standard sieve, and the balance (44%) remaining in the fibrous form. The mixture has a bulk density of about 29 pounds/ cubic foot.

EXAMPLES 4-29 The process of Example 1 is repeated using as dispersant one or more of a variety of monoacid bases and of salts of polybasic acids, which polybasic acids have a first dissociation constant of less than 2X10- formed by reaction of one mole of any of said acids with more than one equivalent of a base selected from ammonium hydroxide and alkali metal hydroxides. The results are shown in Table I. In all of these examples, the initial temperature of the slurry is about 50 C.

TABLE I Densified nitrocellulose Nitro- Particle size, cellulose percent Water, Dispersant (100 total Acetone, parts) Density On Thru A parts parts Composition Parts grade" 1b. lcu. it. mesh mesh Fiber Example:

4 130 70 0. 12 1113-14 43. 7 100 0 130 70 0. 00 HB-14 43. 7 0 100 0 130 70 0. 1113-14 43. 7 0 100 0 130 70 0. HB-14 43. 7 0 100 0 130 70 0. 40 I-IB-14 43. 1 8 92 0 130 70 KzHPO 0. 12 HB-14 43. 7 0 100 0 130 70 (NII4)2IIPO4 0. 12 HB-14 43. 7 0 100 0 130 70 N SiO 0. 10 HB-14 43. 7 0 100 0 130 40. 6 0 100 0 130 43. 7 0 100 0 130 43. 7 0 100 0 130 43. 7 0 100 0 130 43. 7 0 100 0 130 43. 7 0 100 0 130 41. 9 0 100 0 130 41. 2 10 90 0 130 43. 7 0 100 0 130 43. 7 0 100 0 125 47. 5 0 100 0 120 51. 8 0 100 0 100 44. 3 0 100 0 127 41. 2 0 100 0 130 41. 0 0 100 0 130 43. 7 0 100 0 130 43. 7 0 100 0 130 42. 0 0 100 0 *Grade HB-l4 (see Example 1); Grade LC-12, 10w nitrogen, low viscosity grade; Grade HA-5, high nitrogen, high viscosiity grade; Grade PIA-100, high nitrogen, higher viscosity grade; Grade Pyro, 12.6% N, for smokeless powder.

EXAMPLE 30 Example 1 is repeated except that 0.5 part of Gafac? RE610, neutralized to pH=7 by NaOH, is used in place of 0.08 part of Na PO The resulting dry granular nitrocellulose has a bulk density of about 45 pounds/ cubic foot and the particle size distribution, determined by screening through US. Standard sieves, is as follows:

Percent Through 3, on 10 41 Through 10, on 48 Through 40, on 80 6 Through 80 5 EXAMPLE 31 tion as follows:

Percent +3, l0 mesh 24 +10, -40 mesh 58 +40, 80 mesh 10 +80 mesh 8 EXAMPLE 32 Example 31 is repeated except that the dispersant (Gafac RE-6l0) is omitted. Granular densified nitrocellulose is not obtained, and the nitrocellulose forms a gummy mass in the agitated liquid medium.

EXAMPLE 33 Example 31 is repeated except the Gafac RE-610 is not neutralized. Granular nitrocellulose is not obtained and the nitrocellulose forms a gummy mass in the agitated liquid medium.

EXAMPLE 34 A mixture of 125 parts of water, 75 parts of ethyl acetate, 0.3 part of neutralized Gafac R-610, and 100 parts of HB-14 fibrous nitrocellulose (33.3% solids in the system) processed as in Example 1 gives granular densified nitrocellulose free of fibrous material, the bulk density about pounds/cubic foot, and particle size distribution as follows:

Percent 3, +10 mesh 55.4 l0, +40 mesh 37.6 40, mesh 2.3 80 mesh 4.7

What is claimed is:

1. A process for preparing granular nitrocellulose which comprises dispersing fibrous nitrocellulose in an aqueous medium containing about from to 900 parts water per 100 parts fibrous nitrocellulose and a volatile, active solvent for said nitrocellulose in an amount sufficient to soften but not dissolve said nitrocellulose, incorporating in said medium dispersant selected from the group consisting of at least one of (a) ammonium and alkali metal hydroxides and (b) water-soluble salts of polybasic acids having a first dissociation constant of less than 2X10 and more than one equivalent, per mole of said acid, of said hydroxides, and distilling off said solvent.

2. A process of claim 1 wherein the dispersant comprises about from 0.025 parts by weight per 100 parts of nitrocellulose.

3. A process of claim 2 wherein the dispersant comprises about from 0.030.5 part by weight per 100 parts of nitrocellulose.

4. A process of claim 1 wherein the dispersant is ammonium hydroxide.

5. A process of claim 1 wherein the dispersant is an alkali metal hydroxide.

6. A process of claim 5 wherein the dispersant is sodiurn hydroxide.

7. A process of claim 5 wherein the dispersant is potassium hydroxide.

8. A process of claim 5 wherein the dispersant is lithium hydroxide.

9. A process of claim 1 wherein the dispersant is selected from water-soluble salts formed by reaction of polybasic acids that have a first dissociation constant of less than 2X 10- with more than one equivalent, per mole of said acid, of a base selected from ammonium and alkali metal hydroxides.

10. A process of claim 9 wherein the dispersant is selected from ammonium and alkali metal salts of acid organic phosphates.

11. A process of claim 10 wherein the dispersant is selected from water-soluble ammonium and alkali metal salts of acid polyoxyethylene nonylphenol phosphates.

12. A process of claim 10 wherein the dispersant is selected from water-soluble ammonium and alkali metal salts of acid alkyl phosphates.

13. A process of claim 10 wherein the dispersant is selected from water-soluble ammonium and alkali metal salts of acid fluoroalkyl phosphates.

14. A process for the preparation of granular nitrocellulose which comprises agitating about 100 parts by weight of fibrous nitrocellulose in an aqueous bath comprising about from 100-200 parts Water, about from 60- 300 parts acetone and about from 0.02-5.0 parts trisodium phosphate, and continuing agitation while removing substantially all of the acetone until smooth, hardened, densified particles of nitrocellulose are obtained.

References Cited UNITED STATES PATENTS 670,346 3/1901 Luck 260223 1,912,399 6/1933 Norton et al 260-223 2,604,471 7/ 1952 Kridel et a1 260-223 2,946,673 7/1960 Grassie 260-223 2,948,601 8/1960 Grassie 260223 3,284,253 11/1966 Enders et a1. 260223 3,198,645 8/1965 Plunguian 106-170 OTHER REFERENCES Reeves et al.: Industrial and Engineering Chemistry, vol. 39, No. 10, October 1947, pp. 1303-1309.

DONALD E. CZAJA, Primary Examiner R. W. GRIFFIN, Assistant Examiner US. 01. X.R. 

